I haven't tried it, but...I wonder if you could run blackra1n on an already jailbroken phone. If it works, you would have a blackra1n app/icon added to your SpringBoard that you can then use to bootstrap an install of Cydia.
It's certainly worth a shot. However, I'm not sure if the Mac version of blackra1n is a universal binary. It might be Intel only, in which case you'd be out of luck doing it from your G3. You'd have to find another, more modern Windows PC or Mac to run blackra1n from.
-- Nathan